1. Field of the Invention
The present invention relates to a method for feeding an antistatic compound to a polymerization reactor. The invention also relates to a process for the polymerization of olefins carried out in the presence of an antistatic compound fed to the polymerization reactor with the method of the invention.
2. Description of the Related Art
In polymerization processes carried out in continuous, particularly in gas-phase processes for olefin polymerization, there is the need to face up to the formation of polymer agglomerates in the polymerization reactor. The polymer agglomerates involves many negative effects: for example, they can disrupt the discharge of polymer from the reactor by plugging the polymer discharge valves. Furthermore, the agglomerates may also partially cover the fluidization grid of the reactor with a loss in the fluidization efficiency.
It has been found that the presence of fine polymer particles in the polymerization medium favors the formation of polymer agglomerates: these fines may be present as a result of introducing fine catalyst particles or breakage of catalyst and polymer particles within the polymerization medium. The fines are believed to deposit onto and electrostatically adhere to the inner walls of the polymerization reactor and the equipment for recycling the gaseous stream such as, for example, the heat exchanger. If the fines remain active, then the particles will grow in size resulting in the formation of agglomerates, also caused by the partial melting of the polymer itself. These agglomerates when formed within the polymerization reactor tend to be in the form of sheets. Agglomerates can also partially plug the heat exchanger designed to remove the heat of polymerization reaction.
Several solutions have been proposed to resolve the problem of formation of agglomerates during a gas-phase polymerization process. These solutions include the deactivation of the fine polymer particles, the control of the catalyst activity and, above all, the reduction of the electrostatic charge by introducing antistatic agents inside the reactor.
EP 359444 describes the introduction into the polymerization reactor of small amounts of an activity retarder in order to keep substantially constant either the polymerization rate or the content of transition metal in the polymer produced. The process is said to produce a polymer without forming agglomerates.
U.S. Pat. No. 4,803,251 describes a process for reducing the polymer sheeting utilizing a group of chemical additives, which generate both positive and negative charges in the reactor, and which are fed to the reactor in an amount of from about 0.1 to about 25 ppm based on the monomer feed, preferably ethylene, in order to prevent the formation of undesired positive or negative charges.
EP 560035 discloses a polymerization process in which an anti-fouling compound is used to eliminate or reduce the build-up of polymer particles on the walls of a gas-phase polymerization reactor. This anti-fouling compound is preferably selected from alkydiethanolamines, which may be fed at any stage of the gas-phase polymerization process in an amount greater than 100 ppm by weight with respect to the produced (co)polymer. Said anti-fouling compound is capable to selectively inhibit the polymerization on polymer particles smaller than 850 μm, the latter being responsible for fouling problems and polymer sheeting.
It can be pointed out that, when polymerizing olefins by means of a fluidized bed reactor, the prior art literature mainly teaches to introduce antistatic or antifouling compounds directly inside the fluidized polymer bed. As an alternative, these additives are taught to be fed in correspondence of the gas recycle line of the polymerization reactor. Some chemical compounds having antistatic and/or antifouling properties are in a solid form at room temperature: examples of them are hydroxyesters, such as glyceryl monostearate (GMS) and glyceryl monopalmitate. Of course, the high temperatures existing inside a gas-phase polymerization reactor guarantee a quick melting of such solid additives, once they are introduced into the reactor. However, technical problems arise for feeding said solid additives to the reactor, because they cannot flow through the feed lines, as in case of liquid antistatic compounds. It is therefore necessary to arrange a heater device on each feed line, so as to melt the additive and to ensure its flowability through the feed lines. Generally, more lines for feeding an antistatic are present in a gas-phase polymerization plant, so that, notwithstanding the total amount of additive to be fed is low, the arrangement of more heaters on each feed line (jacketed pipes) increases the complexity of the polymerization plant and the energy consumption.
For the above reasons, it would be largely desirable to find an alternative method for introducing antistatic and/or antifouling compounds into a polymerization reactor.